A strategic border crossing and three other towns in western Iraq fell Saturday to the control of ISIS militants, a senior Iraqi security official said.

In addition to their offensives in northern Iraq, the militants have now strengthened their hand in the western province of Anbar, the country's largest geographically, and were controlling Al-Qaim, Rawa, Ana and Husaybah, said the senior official, who's based in Anbar.

Most importantly, the Islamic State in Syria and Iraq, or ISIS, controlled the strategic town of Al-Qaim on the border with Syria, where the enemy fighters enjoy a stronghold, Iraqi security officials said Saturday.

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Together, the four towns are situated along a highway from Syria to Baghdad, heightening possibilities that the militants could now march from the west to lay siege to the Iraqi capital. One of the four towns, Husaybah, is just 100 kilometers, or 62 miles, outside Baghdad.

CNN's Nic Robertson says fighters from Syria are capable of reaching the outskirts of Baghdad in less than four hours.

Iraqi government officials didn't have an immediate comment, other than security officials saying they were expecting troop reinforcements in Anbar.

Several Sunni tribes are aiding and supporting ISIS in Anbar, the senior official said.

Also, the first retinue of U.S. military advisers was expected to arrive soon in Iraq.

Since clashes erupted Friday in Al-Qaim, at least 11 Iraqi soldiers have been killed and 21 more have been wounded. Also, at least 20 militants were killed after Iraqi forces shelled areas from where the extremists launched attacks, two security officials in Ramadi, Iraq, told CNN.

Al-Qaim sits across from Syria's Deir Ezzor province, where ISIS controls at least three towns, including areas near the military airport of Deir Ezzor, which was the headquarters of the military council for rebel battalions, said the Syrian Observatory for Human Rights, an opposition group in London that monitors the Syrian conflict.

Why Al-Qaim is important

"This advancement (in Syria) is considered a very important and strategic step because ISIS has tried to take a complete control over areas in the east of Deir Ezzor in order to reach to the Syrian-Iraqi borders, and then to connect its held areas in both Syria and Iraq with each other," the opposition group said.

Opposite of Al-Qaim is the Syrian town of Al-Bukamal, which is under the control of other Islamist brigades such as Nusra Front, said Rami Abdulrahman of SOHR. ISIS doesn't control that town, he added Saturday.

Iraqi forces were fighting the suspected ISIS militants on at least two fronts: First, they discovered dozens of militants on the Syrian side of the border, security officials said.

At the same time, the support by some Sunni tribesmen for ISIS is proving pivotal in the militants' success, a senior security official in Ramadi told CNN.

If the Sunni tribes do not decide to help and support Iraqi security forces, then it will be very difficult for Iraqi forces to regain the full control of Al-Qaim, the senior official said.

In the meantime, Iraqi forces were waiting for more troops to arrive in Al-Qaim, located about 500 kilometers (about 310 miles) west of Baghdad.

Shiite show of force

Iraq has a long history of brutality between Sunni and Shiite sects, and on Saturday, a warning of renewed conflict between Shiites and the Sunnis supporting ISIS emerged on the streets of Baghdad, where thousands of Shiite militiamen marched in a rally.

The show of force, called a parade, was organized by prominent Shiite cleric Muqtada al-Sadr, who urged unity in Iraq in its fight against ISIS.

The thousands of Shiites wore various security forces uniforms in a march in Baghdad's Sadr City.

They carried rifles, rocket launchers and rocket-propelled grenades.

On the street, they also did a demonstration on planting roadside bombs -- with the armor-piercing bombs hoisted on their shoulders. Those bombs were the Iranian-designed, electronically formed projectiles, which were the scourge of the U.S. war in Iraq during the 2000s.

Even women joined the march, dressed in black and holding handguns.

Only a few years ago, the Shiite militiamen were fighting U.S. forces on the same Baghdad streets.

On Saturday, they demonstrated their readiness for the ISIS fighters.

"ISIS is a terrorist organization created by the United States. They are the enemy of humanity," said one participant who identified himself as a former army colonel. "We are here to free the land for all Sunni and Shiite and everyone."

Another militiaman said he didn't welcome the U.S. initiative to send military advisers to help the government.

"We don't need airstrikes or any external force helping us here," the volunteer said. "We don't want these American military advisers."

A Shiite cleric blamed Prime Minister Nuri al-Maliki for Iraq's latest instability, even though his government favors Shiites.

"The main cause of the security deterioration is the bad management of the prime minister," the cleric said. "Al-Maliki must leave and resign."

Falluja is about 60 kilometers west of Baghdad, and Iraqi forces have so far blocked the militants from marching on the nation's capital. Falluja has been under control of ISIS militants and Sunni tribesmen since January.

The militants in Falluja, however, have been trying to take over Sunni areas close to Baghdad, such as Abu Ghraib and small villages close by, Ramadi security officials told CNN.

Abu Ghraib is a largely Sunni area in the western outskirts of the capital.

Despite the government effort to protect Baghdad, several bomb attacks occurred across the capital city, killing at least seven people and wounding 32 more, officials said Saturday. One of the bomb attacks hit Baghdad's Sadr City, a predominantly Shiite area in the eastern part of the Iraqi capital, police officials in Baghdad told CNN Saturday.

U.S. advisers due to arrive in Iraq soon

The Iraqi government was waiting for the initial group of U.S. military advisers to arrive in Iraq soon, a senior defense official said, as crowds paraded nationwide in a show of unity for the government.

This first detail is expected to be very small, the official said. The total number of U.S. military advisers who will eventually deploy will be about 300.

In addition, some U.S. military personnel already at the U.S. Embassy in Baghdad will be reassigned and become advisers, Pentagon spokesman Rear Adm. John Kirby said.

The first group of advisers will conduct an initial assessment of Iraqi troop capabilities and of what may be needed for a larger group of U.S. advisers, including additional security measures where they may be deployed, a senior defense official said Friday.

Refugee crisis

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Photos:The ISIS terror threat

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Wounded passengers are treated following a suicide bombing at the Brussels Airport on March 22, 2016. The attacks on the airport and a subway killed 32 people and wounded more than 300. ISIS claims its "fighters" launched the attacks in the Belgian capital.

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Syrians gather at the site of a double car bomb attack in the Al-Zahraa neighborhood of the Homs, Syria, on February 21, 2016. Multiple attacks in Homs and southern Damascus kill at least 122 and injure scores, according to the state-run SANA news agency. ISIS claimed responsibility.

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Syrian pro-government forces gather at the site of a deadly triple bombing Sunday, January 31, in the Damascus suburb of Sayeda Zeynab. ISIS claimed responsibility for the attack, according to a statement circulating online from supporters of the terrorist group.

Wounded people are helped outside the Bataclan concert hall in Paris following a series of coordinated attacks in the city on Friday, November 13. The militant group ISIS claimed responsibility for the attacks, which killed at least 130 people and wounded hundreds more.

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Emergency personnel and civilians gather at the site of a twin suicide bombing in Beirut, Lebanon, on Thursday, November 12. The bombings killed at least 43 people and wounded more than 200 more. ISIS appeared to claim responsibility in a statement posted on social media.

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Smoke rises over the northern Iraqi town of Sinjar on November 12. Kurdish Iraqi fighters, backed by a U.S.-led air campaign, retook the strategic town, which ISIS militants overran last year. ISIS wants to create an Islamic state across Sunni areas of Iraq and Syria.

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Syrian government troops walk inside the Kweiras air base on Wednesday, November 11, after they broke a siege imposed by ISIS militants.

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Members of the Egyptian military approach the wreckage of a Russian passenger plane Sunday, November 1, in Hassana, Egypt. The plane crashed the day before, killing all 224 people on board. ISIS claimed responsibility for downing the plane, but the group's claim wasn't immediately verified.

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An explosion rocks Kobani, Syria, during a reported car bomb attack by ISIS militants on Tuesday, October 20.

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Shiite fighters, fighting alongside Iraqi government forces, fire a rocket at ISIS militants as they advance toward the center of Baiji, Iraq, on Monday, October 19.

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Smoke rises above a damaged building in Ramadi, Iraq, following a coalition airstrike against ISIS positions on Saturday, August 15.

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Iraqi men look at damage following a bomb explosion that targeted a vegetable market in Baghdad on Thursday, August 13. ISIS claimed responsibility for the attack.

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In this image taken from social media, an ISIS fighter holds the group's flag after the militant group overran the Syrian town of al-Qaryatayn on Thursday, August 6, the London-based Syrian Observatory for Human Rights reported. ISIS uses modern tools such as social media to promote reactionary politics and religious fundamentalism. Fighters are destroying holy sites and valuable antiquities even as their leaders propagate a return to the early days of Islam.

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An ISIS fighter poses with spoils purportedly taken after capturing the Syrian town of al-Qaryatayn.

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Smoke rises as Iraqi security forces bomb ISIS positions in the eastern suburbs of Ramadi, Iraq, on August 6.

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Buildings reduced to piles of debris can be seen in the eastern suburbs of Ramadi on August 6.

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The governor of the Asir region in Saudi Arabia, Prince Faisal bin Khaled bin Abdulaziz, left, visits a man who was wounded in a suicide bombing attack on a mosque in Abha, Saudi Arabia, on August 6. ISIS claimed responsibility for the explosion, which killed at least 13 people and injured nine others.

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Saudi officials and investigators check the inside of the mosque on August 6.

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Mourners in Gaziantep, Turkey, grieve over a coffin Tuesday, July 21, during a funeral ceremony for the victims of a suspected ISIS suicide bomb attack. That bombing killed at least 31 people in Suruc, a Turkish town that borders Syria. Turkish authorities blamed ISIS for the attack.

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Protesters in Istanbul carry anti-ISIS banners and flags to show support for victims of the Suruc suicide blast during a demonstration on Monday, July 20.

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People in Ashmoun, Egypt, carry the coffin for 1st Lt. Mohammed Ashraf, who was killed when the ISIS militant group attacked Egyptian military checkpoints on Wednesday, July 1. At least 17 soldiers were reportedly killed, and 30 were injured.

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Syrians wait near the Turkish border during clashes between ISIS and Kurdish armed groups in Kobani, Syria, on Thursday, June 25. The photo was taken in Sanliurfa, Turkey. ISIS militants disguised as Kurdish security forces infiltrated Kobani on Thursday and killed "many civilians," said a spokesman for the Kurds in Kobani.

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Residents examine a damaged mosque after an Iraqi Air Force bombing in the ISIS-seized city of Falluja, Iraq, on Sunday, May 31. At least six were killed and nine others wounded during the bombing.

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People search through debris after an explosion at a Shiite mosque in Qatif, Saudi Arabia, on Friday, May 22. ISIS claimed responsibility for the attack, according to tweets from ISIS supporters, which included a formal statement from ISIS detailing the operation.

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Iraqi soldiers fire their weapons toward ISIS group positions in the Garma district, west of the Iraqi capital of Baghdad, on Sunday, April 26. Pro-government forces said they had recently made advances on areas held by Islamist jihadists.

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A member of Afghanistan's security forces stands at the site where a suicide bomber on a motorbike blew himself up in front of the Kabul Bank in Jalalabad, Afghanistan, on Saturday, April 18. ISIS claimed responsibility for the attack. The explosion killed at least 33 people and injured more than 100 others, a public health spokesman said.

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Iraqi counterterrorism forces patrol in Ramadi on April 18.

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Thousands of Iraqis cross a bridge over the Euphrates River to Baghdad as they flee Ramadi on Friday, April 17.

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Yazidis embrace after being released by ISIS south of Kirkuk, Iraq, on Wednesday, April 8. ISIS released more than 200 Yazidis, a minority group whose members were killed, captured and displaced when the Islamist terror organization overtook their towns in northern Iraq last summer, officials said.

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Kurdish Peshmerga forces help Yazidis as they arrive at a medical center in Altun Kupri, Iraq, on April 8.

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A Yazidi woman mourns for the death of her husband and children by ISIS after being released south of Kirkuk on April 8. ISIS is known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts.

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People in Tikrit inspect what used to be a palace of former President Saddam Hussein on April 3.

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On April 1, Shiite militiamen celebrate the retaking of Tikrit, which had been under ISIS control since June. The push into Tikrit came days after U.S.-led airstrikes targeted ISIS bases around the city.

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Iraqi security forces launch a rocket against ISIS positions in Tikrit on Monday, March 30.

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The parents of 19-year-old Mohammed Musallam react at the family's home in the East Jerusalem Jewish settlement of Neve Yaakov on Tuesday, March 10. ISIS released a video purportedly showing a young boy executing Musallam, an Israeli citizen of Palestinian descent who ISIS claimed infiltrated the group in Syria to spy for the Jewish state. Musallam's family told CNN that he had no ties with the Mossad, Israel's spy agency, and had, in fact, been recruited by ISIS.

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Iraqi Shiite fighters cover their ears as a rocket is launched during a clash with ISIS militants in the town of Al-Alam, Iraq, on Monday, March 9.

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Displaced Assyrian women who fled their homes due to ISIS attacks pray at a church on the outskirts of Damascus, Syria, on Sunday, March 1. ISIS militants abducted at least 220 Assyrians in Syria.

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Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on Wednesday, February 4. Al-Kasasbeh's son, Jordanian pilot Moath al-Kasasbeh, was burned alive in a video that was recently released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.

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A Kurdish marksman looks over a destroyed area of Kobani on Friday, January 30, after the city had been liberated from the ISIS militant group. The Syrian city, also known as Ayn al-Arab, had been under assault by ISIS since mid-September.

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Kurdish people celebrate in Suruc, Turkey, near the Turkish-Syrian border, after ISIS militants were expelled from Kobani on Tuesday, January 27.

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Collapsed buildings are seen in Kobani on January 27 after Kurdish forces took control of the town from ISIS.

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Junko Ishido, mother of Japanese journalist Kenji Goto, reacts during a news conference in Tokyo on Friday, January 23. ISIS would later kill Goto and another Japanese hostage, Haruna Yukawa.

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ISIS militants are seen through a rifle's scope during clashes with Peshmerga fighters in Mosul, Iraq, on Wednesday, January 21.

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An elderly Yazidi man arrives in Kirkuk after being released by ISIS on Saturday, January 17. The militant group released about 200 Yazidis who were held captive for five months in Iraq. Almost all of the freed prisoners were in poor health and bore signs of abuse and neglect, Kurdish officials said.

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Smoke billows behind an ISIS sign during an Iraqi military operation to regain control of the town of Sadiyah, about 95 kilometers (60 miles) north of Baghdad, on Tuesday, November 25.

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Fighters from the Free Syrian Army and the Kurdish People's Protection Units join forces to fight ISIS in Kobani on Wednesday, November 19.

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A picture taken from Turkey shows smoke rising after ISIS militants fired mortar shells toward an area controlled by Syrian Kurdish fighters near Kobani on Monday, November 3.

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Iraqi special forces search a house in Jurf al-Sakhar, Iraq, on Thursday, October 30, after retaking the area from ISIS.

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ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23. The United States and several Arab nations have been bombing ISIS targets in Syria to take out the militant group's ability to command, train and resupply its fighters.

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Kurdish fighters walk to positions as they combat ISIS forces in Kobani on Sunday, October 19.

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Heavy smoke rises in Kobani following an airstrike by the U.S.-led coalition on October 18.

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Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.

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Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.

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Alleged ISIS militants stand next to an ISIS flag atop a hill in Kobani on Monday, October 6.

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A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk, Iraq, on Tuesday, September 30.

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Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.

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A elderly man is carried after crossing the Syria-Turkey border near Suruc on Saturday, September 20.

Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.

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Displaced Iraqis receive clothes from a charity at a refugee camp near Feeshkhabour, Iraq, on Tuesday, August 19.

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Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.

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Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.

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Thousands of Yazidi and Christian people flee Mosul on Wednesday, August 6, after the latest wave of ISIS advances.

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A Baiji oil refinery burns after an alleged ISIS attack in northern Selahaddin, Iraq, on Thursday, July 31.

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A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.

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Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.

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More than 1 million Iraqis have fled their homes this year because of conflict, the United Nations refugee agency said Friday. The number is likely to rise as Islamist militants and Iraqi security forces battle for control.

An estimated 800,000 people left Iraq's second-largest city of Mosul after it fell to fighters from ISIS, the International Committee of the Red Cross said. The city has a population of 1.6 million.

ISIS, born from an al Qaeda splinter group and supported by Sunni factions, continues its fierce advance in Iraq.